1. Field of the Invention
This invention relates generally to cryogenic refrigeration systems which have a free-piston, heat pump for lifting heat and are lubricated by gas bearings and more particularly relates to an improved closed loop control system which controls temperature and maintains effective gas bearing operation over a widened range of thermal load applications while permitting energy efficient, piston stroke modulation for controlling cooling power.
2. Description of the Related Art
The applications and uses for refrigeration systems which are capable of cooling to cryogenic temperatures have been expanding for several years. Consequently, designers have sought to improve performance and energy efficiency and reduce the cost of such systems. One important type of cryogenic refrigeration system uses a compressor which has a free piston. These include Stirling and pulse tube free piston cryocoolers. The free piston reciprocates in a cylinder without the restraint of a conventional crank and connecting rod linkage. The piston is driven in reciprocation by one of several types of prime movers, such as a linear electric motor.
One advantage of these free piston cryocoolers is that the stroke of the free piston can be controllably modulated, typically by a closed loop, negative feedback control system, to modulate the cooling power applied by the cryocooler to the work of lifting heat from the low temperature of the thermal load being cooled at the cold end to the ambient temperature at the warm end. The cooling power delivered by a free piston cryocooler is an increasing function of the stroke of the free piston. Therefore, the control system for the cryocooler can control the temperature of the thermal load by controlling the piston stroke to increase or decrease the cooling power over a range of cooling power demand, the term cooling power demand also being known as the thermal load. Piston stroke is controlled by controlling the stroke of and the power input to the prime mover driving the free piston. Energy efficiency can be maximized because the power input to the prime mover can increase and decrease as cooling power demand changes so that the delivered cooling power will equal the cooling power demand, i.e. the cooling power required to maintain the command input temperature.
One such cryocooler is shown in U.S. Pat. No. 5,535,593 to Wu et al. A Stirling cycle cryocooler has its cold finger tip temperature controlled by a closed loop control system which adjusts the stroke of its compressor piston as a function of cryocooler temperature.
The purity of the working gases used in free piston cryocoolers is critical to the operating performance of the cryocoolers. Therefore, ordinary petroleum lubricants are not used for lubrication because they contaminate the working gas. Instead, gas bearing systems are used which circulate a portion of the working gas through the space between the interfacing, relatively sliding components, such as between the piston outer surface and the cylinder surface, between a displacer and the cylinder or between a displacer rod and the piston. The gas operates as a fluid lubricant by applying a force on the interfacing surfaces which moves the surfaces away from contact.
Unfortunately, a gas bearing system requires a minimum gas flow rate which is sufficient to maintain its effectiveness. The gas flow rate through the gas bearing system is an increasing function of piston stroke. Therefore, a minimum piston stroke constraint is imposed on such cryocoolers. Consequently, prior art cryocooler control systems must be designed to confine their range of operation to cooling power outputs between this minimum piston stroke required for gas bearing effectiveness and a maximum piston stroke which avoids damage to the cryocooler. If such a cryocooler encounters operating conditions in which the cooling power demand of the thermal load is less than the cooling power delivered at the minimum piston stroke, the cold finger temperature will not be maintained at the desired set point temperature, but instead will drift to colder temperatures.
One of the most important operating conditions is the temperature of the ambient environment in which the cryocooler is operating. Ambient temperature affects both the rate of heat transfer into the thermal load, such as by conduction through its surrounding insulation, and the rate of heat transfer rejected from the cryocooler into the ambient environment. Although the above limitations on piston stroke are not a problem if the operating conditions are confined to a narrower range, they become a problem if a broader range of operating conditions, such as ambient temperatures, can be anticipated, which includes conditions requiring less cooling power than the cooling power delivered by the heat pump at the minimum piston stroke. Additionally, designing a cryocooler which can operate only over a narrower range of operating conditions, limits the number of applications for which the cryocooler can be used.
It is therefore an object and feature of the invention to provide a cryocooler, including its prime mover and control system, which is capable of operating at a cooling power which is less than the cooling power delivered at its minimum piston stroke while still maintaining both its piston stroke at the minimum stroke necessary for proper gas bearing lubrication and the temperature of the thermal load at the set point temperature.
Another object and feature of the invention is to provide a cryocooler system which can take advantage of the energy efficiency of piston stroke modulation and is also capable of operating over a broader range of cooling power demands and therefore over a broader range of operating conditions, for example over a broad range of ambient temperature such as from −40° C. to +70° C., and for the same reason may be applied to a more extensive variety of applications and uses.